This invention relates generally to ignition systems, and more specifically to a method and apparatus for generating a sustained arc at a sparking device.
Many types of spark ignition systems are known in the art. Such prior art ignition systems generally create sparks of very short duration but with relatively high peak power. In two predominant system types, substantially all of the energy to be discharged is stored in either an inductance coil or a capacitor and then discharged rapidly to create the spark. The later system type, called Capacitive Discharge (CD) ignition is more prevalent than the former type for high-energy spark applications because capacitors are more volumetrically efficient at storing energy than inductors in most practical circumstances.
In either type of system, the typical sequence of operation is: charge an energy storage device; discharge that energy rapidly through a switch to a sparking device; and wait for a predetermined period of time before repeating the charge cycle to generate successive sparks. These three events have relatively different times associated with them. Generally, the charge cycle is accomplished in a few ten""s of milliseconds. The discharge event is instantaneous in comparison, lasting only a few hundred microseconds. The inter-spark time delay, on the other hand, is typically several times longer than the charge time. The short spark duration is a result of the RC (Resistance*Capacitance) time constant of the discharge circuit. Once ionized, the plasma presents a very low resistance, on the order of tens to hundreds of milliohms, so even for large values of capacitance the time constant, R*C, is still short. The time constant is defined as the time required for 63.2% of the initial voltage stored in the capacitor to be depleted. The energy (E) stored in a capacitor can be defined by the equation E=xc2xd*C*V2, so once the voltage has fallen by 63.2%, (i.e., to 36.8% of its initial value), only 13% of the energy initially stored in the energy storage device remains. In other words, the spark is nearly over after just one time constant.
It is desirable to increase the energy delivered by the spark to the fuel mixture in order to promote ignition. It may also be beneficial to lengthen the duration of the spark because there is also a thermal transfer time constant associated with heating the fuel droplets. For an extremely short duration spark, the spark may terminate before sufficient thermal transfer can be completed such that ignition fails to occur.
Once a plasma has been formed, it can be heated by forcing a current (I) through its resistance (R). The power (P), delivered substantially as heat, is P=I2*R, and the total energy delivered is the accumulation, or integration, of that power over time. The requirements to ionize a spark generating device and to sustain a plasma at that same device are very different. Ionization requires a high voltage to overcome the circuit discontinuity presented by the gap of the sparking device, but only a small current is required, and for a short time. Conversely, sustaining the plasma requires a lower voltage because the ionized plasma has a very low impedance, but a higher current is needed, and for a significantly longer time, to transfer any substantial amount of energy to the arc to promote ignition.
Circuits which strike (initiate) an arc and subsequently maintain it with additional energy input are known in the art. U.S. Pat. No. 3,788,293 discloses a circuit in which a sparking device is ionized by a pulse from a high voltage ignition coil associated with a transformer, and then sustained by the discharge of a capacitor also connected to the sparking device. The current from the capacitor does not have to pass through the transformer. Similarly U.S. Pat. No. 3,835,830 describes a circuit which first generates an extra-high voltage pulse to strike an arc, and then maintains the current through the spark generating device using the discharge of a high voltage capacitor which delivers its current through a series connected winding of the same transformer that generates the initial extra-high voltage pulse.
Both of these circuits suffer from certain disadvantages. For example, in both of these circuits a high voltage is present at the sparking device before the intended sparking time. Because of this condition, in many applications, these circuits will not work reliably. They only work with high tension sparking devices, and are rendered inoperable by fouling which presents a shunt impedance across the gap of the sparking device. Low-tension sparking devices which inherently present a shunt impedance before ionization, and high tension plugs, if severely fouled with deposits that create a conductive path, will not always function correctly with these circuits.
Following ionization, both of the aforementioned circuits deliver energy to the plasma from a capacitor. Thus, the flow of energy is a decaying function; most of the energy is delivered quickly following ionization, after which the flow of energy to the plasma gradually diminishes until it is zero. Thus, neither of these circuits is capable of delivering a sustained current to the spark-generating device.
It is a general object of the invention to provide an improved apparatus for generating a sustained arc at a sparking device. It is a more specific object to provide an apparatus for initiating and sustaining a plasma across the gap of a spark generating device in order to deliver sufficient energy to a fuel mixture to ensure its ignition.
It is another object of the invention to eliminate the large tank capacitor employed in conventional high-energy CD ignition systems, while providing increased energy to the sparking device. It is a related object to provide such a device wherein the increased energy is provided by pumping energy for a longer time, rather than by increasing the value of the tank capacitor to thereby increase the stored energy.
It is another object of the invention to provide improved ignition by lengthening the duration of the spark while maintaining its energy and heat at a high level.
It is a related object of the invention to control the total energy in a spark by controlling the time duration of the sustained pumping of energy into the plasma.
It is another object of the invention to vary the level of pumping of energy through the plasma during a particular cycle to shape the electrical waveform, and consequently affect the physical characteristics of the arc to improve ignition. It is a related object of the invention to control the total energy in a spark by controlling the average current during the interval of sustained pumping of energy into the plasma.
It is yet another object of the invention to reduce wear on the spark-generating device by controlling the timing of pumping of energy to coincide with the varying physical position of the plasma arc.
It is still another object to provide ignition control adaptive to the sensed or predicted needs of the combustor by controlling the total energy on a spark-by-spark basis, responsive to the immediate conditions that affect the probability of successful ignition.
The present invention accomplishes the foregoing and other objectives by providing an apparatus which generates an ionizing pulse to a spark generating device and, as soon as a plasma forms across an air gap of the device, begins controlled pumping of energy into the arc to sustain it, heat it, and deliver energy sufficient to cause ignition.
In accordance with one aspect of the invention, the pumping of energy is an active process rather than the prior passive process of simply dumping a previously stored quantity of energy.
In accordance with another aspect of the invention, the same energy converter pumps energy to the spark generating device to sustain the arc and provides the energy for the ionizing pulse that starts the arc.
It is another aspect of the invention to utilize dual intermittent converters operating alternately to ensure the uninterrupted flow of energy to the arc so that it does not extinguish during the intermittencies of either converter.
It is another aspect of the invention that the delivery of energy to the sparking device is not a decaying function such as would be provided by a conventional capacitive discharge.
It is a related aspect of the invention to deliver the energy according to a predefined function that is not a constant during the period of the pumping of energy.
It is yet another aspect of the invention to respond to a commanded level or total quantity of energy by varying the pumping of energy. In a related aspect of the invention, the apparatus contains a predefined response pattern. In yet another related aspect, the response pattern is created by sensing the instant conditions and calculating the appropriate level and quantity of energy.
In accordance with another aspect of the invention, the sustaining of the arc is terminated at the time when a sensing apparatus determines that ignition has occurred.
In accordance with yet another aspect of the invention, the pumping of large amounts of energy into the arc is deferred until the center of the plasma has moved away from the tip of the spark generating device to reduce wear on the spark generating device.
These and other features and advantages of the invention will be more readily apparent upon reading the following description of the preferred embodiment of the invention and upon reference to the accompanying drawings wherein: